A primer on the latest in the search for Flight 370

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Malaysian plane sonar search cut short

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Bluefin-21 may be able to be reprogrammed to go deeper, U.S. Navy official tells CNN

Underwater probe back in water in hunt for Flight 370

The first effort to deploy a deep-sea probe ended prematurely

Surface search efforts are winding down as authorities focus their efforts underwater

The search for Malaysia Airlines Flight 370 is now in its 40th day, and searchers have yet to turn up a piece of the plane. Here's the latest to catch you up on the search efforts:

What's the latest with the search?

The Bluefin-21 probe being used to search for the missing jetliner in the deep waters of the Indian Ocean returned to duty Tuesday after its first mission ended prematurely. The probe surfaced early in its first deployment after encountering waters that exceeded its current 4,500-meter (14,764-foot) maximum depth. What was supposed to be a 20-hour mission ended in less than eight, according to a source, and found nothing. An official with the U.S. Navy told CNN's "The Lead with Jake Tapper" that engineers believe the Bluefin-21 probe can go as deep as 5,000 meters (16,404 feet) with some software adjustments.

Does that mean something's wrong with the probe?

Not at all. In fact, the probe did exactly what it's programmed to do when the ocean floor dips below its maximum depth, said David Gallo, director of special projects at the Woods Hole Oceanographic Institution. "The vehicle's tracking the floor, so when the floor dives, so does the vehicle. And the vehicle goes, 'Uh oh, I'm not supposed to be here' and punches up," he said.

AUV aborted mission, returns early

Relatives of passengers from Malaysia Airlines Flight MH370 console each other outside the Malaysia Airlines office in Subang, Malaysia, on Thursday, February 12. Protesters demanded that the airline withdraw the statement made in January that all the passengers aboard the plane are dead. The plane, which disappeared on March 8, has not been found.

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The search for Malaysia Airlines Flight 37048 photos

A policewoman watches a couple whose son was on board the missing Malaysia Airlines Flight 370 cry outside the airline's office building in Beijing after officials refused to meet with them on June 11, 2014. The search for the missing plane has been ongoing since early 2014.

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The search for Malaysia Airlines Flight 37048 photos

Members of the media scramble to speak with Azharuddin Abdul Rahman, director general of Malaysia's Civil Aviation Department, at a hotel in Kuala Lumpur, Malaysia, on May 27. Data from communications between satellites and missing Malaysia Airlines Flight 370 was released the day before, more than two months after relatives of passengers say they requested that it be made public.

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Operators aboard the Australian ship Ocean Shield move Bluefin-21, the U.S. Navy's autonomous underwater vehicle, into position to search for the jet on April 14.

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A member of the Royal New Zealand Air Force looks out of a window while searching for debris off the coast of western Australia on April 13.

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The Echo moves through the waters of the southern Indian Ocean on April 12.

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A Royal Australian Air Force AP-3C Orion, on a mission to drop sonar buoys to assist in the search, flies past the Australian vessel Ocean Shield on April 9.

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A relative of a missing passenger cries at a vigil in Beijing on April 8.

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Australian Defense Force divers scan the water for debris April 7, in the southern Indian Ocean.

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A towed pinger locator is readied to be deployed April 7 off the deck of the Australian vessel Ocean Shield.

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A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean during search operations on April 4.

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A member of the Japanese coast guard points to a flight position data screen while searching for debris from the missing jet on April 1.

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A woman prepares for an event in honor of those aboard Flight 370 on March 30, in Kuala Lumpur, Malaysia.

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A Royal New Zealand Air Force member launches a GPS marker buoy over the southern Indian Ocean on March 29.

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The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on March 28, after other relatives left en masse to protest the Malaysian government's response to their questions.

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A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on March 27.

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Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27.

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People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27.

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Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight on March 24 in Kuala Lumpur. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived."

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Grieving relatives of missing passengers leave a hotel in Beijing on March 24.

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Ground crew members wave to a Japanese Maritime Defense Force patrol plane as it leaves the Royal Malaysian Air Force base in Subang, Malaysia, on March 23. The plane was heading to Australia to join a search-and-rescue operation.

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A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on March 22.

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A Chinese satellite captured this image, released on March 22, of a floating object in the Indian Ocean, according to China's State Administration of Science. It is a possible lead in the search for the missing plane. Surveillance planes are looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia.

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Satellite imagery provided by the Australian Maritime Safety Authority on March 20, shows debris in the southern Indian Ocean that could be from Flight 370. The announcement by Australian officials that they had spotted something raised hopes of a breakthrough in the frustrating search.

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Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight.

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A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on March 19.

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A relative of a missing passenger tells reporters on March 18 in Beijing about a hunger strike to protest authorities' handling of information about the missing jet.

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U.S. Navy crew members assist in search-and-rescue operations March 16, in the Indian Ocean.

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Members of the Chinese navy continue search operations on March 13. The search area for Flight 370 has grown wider. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, efforts are expanding west into the Indian Ocean.

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A Vietnamese military official looks out an aircraft window during search operations March 13.

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Malaysian air force members look for debris on March 13 near Kuala Lumpur.

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Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12.

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Indonesian air force officers in Medan, Indonesia, examine a map of the Strait of Malacca on March 12.

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A member of the Vietnamese air force checks a map while searching for the missing plane on March 11.

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Vietnam air force Col. Le Huu Hanh is reflected on the navigation control panel of a plane that is part of the search operation over the South China Sea on March 10.

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A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported March 8. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10.

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A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews before returning to search for the missing plane March 9, in the Gulf of Thailand.

Italian tourist Luigi Maraldi, who reported his passport stolen in August, shows his current passport during a news conference at a police station in Phuket island, Thailand, on March 9. Iranians Pouri Nourmohammadi and Delavar Seyed Mohammad Reza were identified by Interpol as the two men who used stolen passports to board the flight. But there's no evidence to suggest either was connected to any terrorist organizations, according to Malaysian investigators. Malaysian police believe Nourmohammadi was trying to emigrate to Germany using the stolen Austrian passport.

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Vietnamese air force crew stand in front of a plane at Tan Son Nhat airport in Ho Chi Minh City on March 9 before heading out to the area between Vietnam and Malaysia where the airliner vanished.

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Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9.

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The Chinese navy warship Jinggangshan prepares to leave Zhanjiang Port early on March 9 to assist in search-and-rescue operations for the missing Malaysia Airlines flight. The Jinggangshan, an amphibious landing ship, is loaded with lifesaving equipment, underwater detection devices and supplies of oil, water and food.

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Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9. The vessel is carrying 12 divers and will rendezvous with another rescue vessel on its way to the area where contact was lost with Malaysia Airlines Flight 370.

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The rescue vessel sets out from Sanya in the South China Sea on March 9.

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Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8.

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A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8.

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Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8.

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Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference on March 8 at a hotel in Sepang. "We deeply regret that we have lost all contacts" with the jet, he said.

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EXPAND GALLERY

But what if the plane is in deeper waters?

"Well, if the Bluefin cannot bring back the kind of sonographic images they want and the information that they want, then they're going to have to move on to the next level of vehicles with names like Alvin, Remora and the Sea Dragon, and those can go deeper," said CNN aviation consultant Mary Schiavo. "The Sea Dragon can go down to 4.4 miles, or 7,000 meters. And that's the next step that you have to do, is go down to the level in a different kind of vehicle."

Why aren't they listening for pings from the plane's 'black boxes' anymore?

Because the batteries powering the boxes' locator beacons are probably dead, according to the manufacturer and other experts. The batteries were supposed to last at least 30 days, and the plane has now been missing for 39 days. The batteries could have continued powering the beacons for a few more days but almost certainly have run out by now. Searchers using devices to listen for the pings went six days without hearing anything, so they are now focused on the underwater search.

What happens if they find the black boxes?

The flight data recorders, or FDRs, would be transferred to fresh water and then dried before the data they contain would be extracted, Schiavo said. "Then they'll discover on the FDR what they're dealing with and how much of the wreckage they really have to bring up to solve the mystery."

But don't hold your breath. The Flight 370 search is often compared to the hunt for wreckage from Air France Flight 447, which plunged into the South Atlantic Ocean in 2009, killing all 228 people aboard. It took investigators nearly two years to recover the black boxes in that case.

Is the surface search continuing?

It is, but maybe not for much longer. Eleven military aircraft, three private planes and 11 ships participated in the surface search Wednesday about 2,087 kilometers (1,297 miles) northwest of Perth, Australia, according to the country's Joint Agency Coordination Centre. The center's director, retired Air Marshal Angus Houston, said Monday that the surface search is likely to end in the next few days.

Just Watched

Bluefin-21 'slower than you can walk'

Searchers found an oil slick in the area over the weekend and are shipping a 2-liter sample back to Australia for analysis. If it's oil typically used in aircraft, the slick could be an important lead. But it may not be. A slick found in the early days of the search for Flight 370 turned out to be fuel oil from a freighter.

Any other new details?

A U.S. official told CNN on Monday that co-pilot Fariq Abdul Hamid's cell phone was on and made contact with a cell tower in Malaysia about the time the plane disappeared from radar.

However, the U.S. official -- who cited information shared by Malaysian investigators -- said there was no evidence Fariq had tried to make a call.

The details do appear to reaffirm suggestions, based on radar and satellite data, that the plane was off course and was probably flying low enough to obtain a signal from a cell tower, the U.S. official said.